Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are instruments which typically use a sharp tip to characterize the surface of a sample down to nanoscale dimensions. The term nanoscale as used for purposes of this disclosure refers to dimensions smaller than one micrometer. SPMs monitor the interaction between the sample and the probe tip. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular site on the sample, and a corresponding map of the site can be generated. Because of their resolution and versatility, SPMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research. In other applications, SPM systems may be used for measuring nanomechanical properties of a sample.
The probe of a typical SPM includes a very small cantilever fixed to a support at its base and having a sharp probe tip extending from the opposite, free end. The probe tip is brought very near to or into contact with a surface of a sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with an extremely sensitive deflection detector such as an optical lever system as described, for example, in Hansma et al. U.S. Pat. No. RE 34,489, or some other deflection detector such as strain gauges, capacitance sensors, etc. The probe is scanned over a surface using a high resolution three-axis scanner acting on the sample support, the probe, or a combination of both. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or other surface properties or nanomechanical properties of the sample.
Different SPM probe tip shapes are used for a variety of applications. One type of tip shape that is commonly used for measuring the height of certain nanoscale features, for testing material properties (e.g. elastic modulus), or for manipulating very small objects is a pointed shape (e.g. parabolic) having a relatively simple profile. To image or measure surface features such as vertical sidewalls and undercut regions, and to take critical dimension (CD) measurements, SPMs utilize more complex probe tip shapes, such as boot-shaped or inverted mushroom-shaped probe tips, some of which may have one or more protrusions along the scanning direction.
SPMs may be configured to operate in a variety of modes, including modes for measuring, imaging, or otherwise inspecting a surface, and modes for measuring nanomechanical properties of a sample. In a contact mode operation, the microscope typically scans the tip across the surface of the sample while maintaining a constant probe-sample interaction force. In an oscillation mode of operation, sometimes referred to as tapping mode, the tip of the SPM is oscillated while interacting with the sample at or near a resonant frequency of the cantilever of the probe. The amplitude or phase angle of this oscillation is affected by the probe-sample interaction, and changes in the oscillation are sensed.
As the probe is scanned over the surface of the sample, a probe positioning control system monitors the interaction of the probe with the sample surface such as, for example, deflection of the cantilever (in the case of contact mode), or changes in the oscillation amplitude or phase angle (in the case of oscillating mode). The control system adjusts the probe's position (or average position in the case of oscillating mode) relative to the sample to maintain a constant probe-sample interaction. The position adjustment thus tracks the topography of the sample. In this way, the data associated with the position adjustment can be stored, and processed into data that characterizes the sample. This data can be used to construct an image of the inspected sample's surface, or to make certain measurements of selected surface features (such as, for example, a height of the feature).
The probe position adjustment is effected by a cantilever positioning actuator that is driven by a driving circuit. Various technologies for cantilever actuators are known, including piezoelectric and magnetic transducers. The driving circuit generates a probe positioning signal, and amplifies the probe positioning signal to produce a driving signal that is applied to the actuator. The driving signal continuously repositions the probe's separation distance from the sample to track an arbitrary topography of the sample's surface. Accordingly, the driving signal has a bandwidth from zero hertz to a frequency associated with the maximum operating bandwidth of the SPM, which corresponds to the maximum speed at which the probe can track the topography of the surface of the sample.
Inspection of a sample, in particular, the inspection of a wafer sample entails positioning of the probe at a precise location on the sample so that certain parts of the wafer can be scanned and a topographic image of the feature of interest on the wafer can be obtained. To do this, wafer registration points are identified and an x-y coordination system or navigation coordinate system is built into the SPM that positions the probe in relation to these registration points.
Defect monitoring and review of the quartz thickness in the semiconductor manufacturing industry for today's advanced integrated circuits is done to ensure the fidelity of the wafer print. It is important that quartz depth differences be tightly controlled in mask production. Thus, a SPM has sub nanometer depth metrology precision over a range from angstroms to microns and is used for metrology in the vertical or depth/height direction, metrology for line or trench width, sidewall angle, line edge roughness (LER), line width variation (LWV) and sidewall roughness (SWR). In addition, SPM provides key three-dimensional, topographical information for defect review. SPM thus permits a more accurate classification of defects and thus a faster time to solution for eliminating the defects.
For both metrology and defect review, image placement accuracy is a key factor in the total measurement uncertainty. Conventional automated SPM systems employ an optical microscope and video imaging system to scan and capture wafer feature information relative to unique locations on a wafer. This feature information is used to define a wafer based coordinate system, which is then correlated to the SPM system coordinate system for navigation to features of interest for subsequent imaging and inspection with the SPM probe tip.
Various registration calibration schemes are employed; wherein a single point, or a plurality of points, on the wafer is scanned and captured to define the origin and ordinate axes of the wafer coordinate system, as well as, the rotation of the wafer from loading onto the stage and the residual orthogonality and scale errors present. Contributing to navigation inaccuracy are uncertainties arising from scanning the features used as the baseline marks in the definition of the origin and deskew points in wafer registration locations. For example, the error resulting associated with the definition of the origin location can result in a systematic bias in locating other features on the wafer at least equal to the error term in the origin definition.
Two primary sources of location uncertainty exist in wafer registration for all SPM systems that employ an optical microscope for navigation. The first source is the minimum pixel resolution of the video capture system. Even when inter-pixel interpolation is used, non-negligible location accuracy errors can cause future unacceptable navigation inaccuracy to features of interest due to the inaccuracy in the video capture system definition of wafer feature location. The second source is a result of the physical offset between the probe tip and the center of the video field of view. This physical offset must be calibrated and corrected for in any navigational moves that bring the probe tip in contact with the region of interest of the sample surface that was previously defined/imaged in the video capture system. While there are well known methods to calibrate and correct these offsets, there still remain non-negligible errors from this offset calibration that can easily exceed the required navigation accuracy requirements of many applications. In addition, conventional video based wafer registration systems have the added issues of excessive scanning time due to the added complexity of obtaining acceptable navigation accuracy and the mechanical degradation of the probe tips.
What is needed in the industry is a system and method that negates the navigation inaccuracy to features of interest, while improving scanning time and limiting probe degradation in scanning probe microscopy.